Catalytic differential calorimetric gas sensor

ABSTRACT

A catalytic differential gas combustible microcalorimeter sensor for monitoring the exhaust gas conversion for monitoring the exhaust gas conversion efficiency of a catalytic converter. The catalytic calorimetric sensor disclosed includes a sol-gel processed washcoat and sol-processed catalytically active metal particles. Sol-gel processing creates a washcoat with high surface area and controlled porosity which increases the sensitivity, durability, and reproducibility of the resultant sensor.

This is a divisional of application Ser. No. 08/311,299, filed on Sep.23, 1994 now U.S. Pat. No. 5,707,148.

TECHNICAL FIELD

The present invention is concerned with diagnostic methods and devicesfor monitoring exhaust gases generated from automotive engines.

BACKGROUND OF THE INVENTION

The Environmental Protection Agency (EPA) and the California AirResources Board (CARB) have implemented stringent diagnosticrequirements for automotive emissions. As part of their requirements,CARB has mandated on-board monitoring of the exhaust gas conversionefficiency of catalytic converters, under its On-Board Diagnostics phase2 (OBD-II) plan.

Exhaust gas constituents (EGC) sensors have been proposed as an answerto the new regulation. One such potential EGC sensor is the catalyticcalorimetric sensor.

In a catalytic calorimetric sensor combustible gases (such ashydrocarbons HC, carbon monoxide CO, hydrogen H₂, etc.) are oxidizedwith the help of a catalytic layer. The generated heat, measured as theincrease in substrate temperature, results in an electrical outputsignal proportional to the amount of combustible gases present in thegas mixture.

Catalytic calorimetric gas sensors typically operate in the 250° to 500°C. temperature range, making them in principle applicable for automotiveapplications. Although generally of lower sensitivity thansemiconducting-type gas sensors, catalytic calorimetric sensors appearto be considerably more stable and faster responding. However, existingcatalytic calorimetric sensors have been investigated and found notsuitable for automotive use because of application-oriented limitations.Such limitations have included a lack of sensitivity, restrictivedetection limits and response time, susceptibility to flow andtemperature variation.

These disadvantages of the prior art devices combine to limit theusefulness and applicability of catalytic calorimetric gas sensors.

U.S. Pat. No. 4,355,056 discloses a method of manufacturing adifferential thermocouple combustible sensor which makes the sensorrelatively insensitive to sulfur poisoning. The catalytic thermocouplejunction of a catalytic/non-catalytic junction pair is formed by coatingit with a gel to increase the surface area and then with achloroplatinic acid solution to make it catalytic. The catalyticjunction is then treated with H₂ S to achieve a high catalyst surfacearea. In this patent, the noble metal catalyst is applied from asolution, which results in large particle sizes and an accordingly smallnumber of catalytic sites, the resulting sensor lacks sensitivity.

The prior art suffers from a lack of sensitivity. There thus exists aneed for a more sensitive gas sensor which also exhibits durability.

SUMMARY OF THE INVENTION

The present invention relates to a sensitivity-enhanced catalyticcalorimetric sensor.

The present invention discloses a catalytic calorimetric sensorcomprising: a substrate, a temperature measuring layer and a sol-gelprocessed catalytic layer.

The invention also discloses a catalytic calorimetric sensor comprising:a substrate, a temperature measuring layer and a catalytic layer whichcomprises a sol-gel processed washcoat and a plurality of catalyticallyactive metal particles loaded thereon.

An alternative embodiment of the present invention teaches a catalyticcalorimetric gas sensor, comprising: a substrate, a temperaturemeasuring layer and a catalytic layer which comprises a sol-gelprocessed washcoat and a plurality of sol-gel processed catalyticallyactive metal particles deposited on the washcoat.

The present invention also discloses a silicon micromachining method forproducing a catalytic calorimetric gas sensor to yield a highlyreproducible and sensitive combustible gas sensor.

Lastly, the present invention discloses a method to maximize depositionof the catalytically active metal particles in the pores of thewashcoat. This method reduces agglomeration of the metal particles whilemaking high surface area metal particles available for catalyticreactions.

Sol-gel processed alumina/silica washcoats are beneficial for use withsensors due to the high surface area and controlled porosity that can beachieved.

The use of a sol-gel processed catalytically active metal particlesresults in a catalyst comprising smaller metal particles of betteruniformity than those provided from conventional coating systems, suchas sputtering and the like.

It is an object of the present invention to provide a catalyticcalorimetric gas sensor, where some or all of the catalytic layer isprocessed using a sol-gel technique to create a sensor having anincreased number of active catalytic sites for catalytic oxidation ofthe combustible gas molecules.

It is also an object of the present invention to provide asensitivity-enhanced calorimetric gas sensor using a sol-gel techniqueto process some or all of the catalytic layer.

It is another object of the present invention to provide a catalyticcalorimetric gas sensor that is more durable and more easy tomanufacture.

It is a further object of the present invention to provide a method forfabricating catalytic calorimetric sensors with lower power consumptionat potentially lower manufacturing costs using silicon micromachining.

The above objects and other objects, features and advantages of thepresent invention are readily apparent from the detailed description ofthe best mode for carrying out the invention when taken in connectionwith the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic representation of the catalytic calorimetricsensor utilized in the present invention;

FIG. 2 is a perspective view of the microcalorimeter design used in theinvention herein described; and

FIG. 3 is a cross-section of FIG. 2 taken along lines 4--4.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention generally relates to the application of a sol-gelprocessed, high-surface area alumina and/or silica washcoat andcatalytically active metals impregnated thereon to fabricate thecatalytic layer of a combustible gas sensor. Sol-gel processedalumina-silica materials are beneficial in sensor applications becausesuch materials can be processed easily and have desired properties suchas high-surface area and controlled porosity resulting in a sensor withincreased sensitivity and durability.

Sensitivity is determined by two factors, the rate of gas diffusion andthe rate of oxidation. In this two step process, the gas molecules aretransported by diffusion to the catalytic layer, and then oxidized.Sol-gel processing provides a way to increase the number of catalyticactive sites and thus increase the rate of oxidation for certainspecified exhaust gases. However, in the beginning stages of sensorusage, the increased number of catalytic sites may not drasticallyincrease the sensitivity of the sensor. It is believed that initially,once a given amount of active sites are created, the marginal utility ofeach subsequent active site decreases. One theory is that during thisinitial period, because of the large number of active sites created, therate limiting step becomes the rate of gas diffusion and not the rate ofoxidation. Nonetheless, over a period of years the number of catalyticsites decreases as a result of poisoning of the catalyst and thermalsintering. Thus by increasing the number of catalytic sites produced,the enhanced level of sensitivity will be sustained over the course ofoperation. Accordingly, the durability and life of the sensorsignificantly increases through the use of sol-gel processed catalyticsensors.

It is further believed that the instant solgel technique provides acatalytic layer with an increased number of active catalytic sitescompared to those provided from conventional coating systems. This thenmay allow the formed coating to be thinner, resulting in a catalyticcalorimetric sensor having an enhanced sensitivity.

A schematic diagram of a catalytic calorimetric sensor is shown inFIG. 1. It consists of a substrate 30. On top of this substrate is alayer to measure the temperature 32. The temperature measuring devicewhich comprises the temperature measuring layer can be selected from thegroup consisting of a thermocouple, a temperature dependent metalresistor, a temperature dependent semiconductor resistor, a p-n junctionsemiconductor and a thermopile. This layer can be made by sputtering,screen printing, sol-gel process, etc. Additionally, a catalytic layer34 is placed on top of the temperature measuring layer to enable theoxidation of combustible gases in the 300°-500° C. temperature range.The substrate should be as thin as possible. The substrate shouldpreferably have a thickness in the range between 500 to 1000 nm. Thesubstrate should also be comprised of materials with a low thermalconductivity, such as a ceramic or a silicon micromachined structure.Examples of suitable ceramic materials include aluminum oxide, siliconoxide, polysilicon, silicon nitride or combinations thereof.

Below we will also discuss the most preferred embodiment, a differentialmicrocalorimeter structure as shown in FIGS. 2 and 3. It is adifferential microcalorimeter in which one membrane is covered with acatalytic layer and the other membrane acts as a reference to compensatefor temperature fluctuations in the gas. FIG. 2 is a perspective view ofone embodiment of a catalytic differential calorimetric sensor having asilicon frame 40 with two membranes 44 and 45 with temperature measuringlayers 48 and 49 placed on top of the membrane. The temperaturemeasuring layers are covered by a passivation layer 41. A catalyticlayer 46 is placed on top of membrane 45. The temperature measuringlayer 49 on membrane 44 is used to measure the temperature of thesurrounding gas, while the temperature measuring layer on membrane 45measures the additional heat generated by the catalytic layer. Bothmembranes are thermally insulated from each other, because bothmembranes have a low thermal conductivity. A cross-sectional view of thesensor of FIG. 2 taken along lines 4-4 is shown in FIG. 3. The variousmethods of operating differential calorimetric sensors are known andhave been described in, for example, CALORIMETRY FUNDAMENTALS ANDPRACTICE by W. Hemminger (1984), herein incorporated by reference.

The membranes 44 and 45 can be made of silicon nitride. The preferredembodiment includes membranes made of a composite of silicon nitride andsilicon oxide layers, most preferably silicon nitride, silicon oxide andsilicon nitride layers.

Aluminum oxide can be deposited on top of the membrane structure toimprove the adhesive properties of the catalytic layer on the membrane.The resultant membranes have a low thermal conductivity and are tensileso they stay flat over a wide temperature range without buckling.

Although microfabricated catalytic microcalorimeters (FIGS. 2 and 3) areknown, the present invention provides a chemical and more effectivemeans of fabrication using the sol-gel process. Metals are traditionallyloaded on the membrane by sputtering; however, this technique canproduce only a limited number of active sites for catalytic oxidizationof the combustible gas molecules. This limited number of activecatalytic sites might reduce the sensitivity of a catalytic calorimetricgas sensor. To increase the sensitivity and durability of the sensor forautomotive applications, this invention describes sol-gel methods whichincrease the number of catalytic sites without increasing the size ofthe device. The number of catalytic sites are increased by the use ofsol-gel processed alumina/silica based materials which provide ahigh-surface area washcoat having controlled porosity with narrowpore-size distribution.

In addition to the sol-gel processed washcoat, the present inventiondiscloses a method to maximize deposition of the catalytically activemetal particles in the pores of the washcoat. This method reducesagglomeration of the metal particles available for catalytic reactions.Accordingly, the present invention directed efforts to preparingcontrolled, small metal particles, using a technique disclosed by Schmidin Chem. Rev. 92 (1992) 1709-1727. With Schmid's technique, small nobleparticles were stabilized by small organic molecules such as sulfanilicacid salts. Another method for preparing controlled size catalyticallyactive metals includes impregnation, whereby the particles are preparedon the washcoat itself. These small noble metal particles in conjunctionwith high-surface area alumina-based membranes provide a substantiallylarger number of catalytic sites for calorimetric sensors as compared toconventional fabrication routes.

As used herein, the term washcoat refers to the supporting material onwhich the catalytically active metal particles are loaded. The termsubstrate refers to the material supporting the temperature measuringlayer and the catalytic layer. The substrate is generally a ceramic 30or a silicon micromachined structure consisting of a silicon frame 40and membranes 44 and 45.

There are several metals which act as catalytically active metals,including but not limited to Fe, Cu, Co, Cr, Ni, Mn, Zn, Cd and Ag andmixtures thereof; however, the preferred active metals are generallynoble metals. Noble metals are preferred in large part due to theirstability towards catalyst poisons. Noble metals include but are notlimited to platinum, palladium, silver, gold, ruthenium, rhodium,osmium, iridium and mixtures thereof.

In one washcoat embodiment, alumina sols were prepared by hydrolyzingaluminum alkoxides in water followed by peptization in the presence ofdilute mineral acid. The sol was concentrated to a gel which was thenheated at 250° C. and 600° C. in a nitrogen atmosphere. An aluminawashcoat was then prepared from the alumina sols and showed a surfacearea of 230 m² /g and a pore size of 45 Å.

Another washcoat embodiment includes preparation of alumina/silicawashcoats.

Alumina/sicila washcoats were prepared from a mixture of alumina andsilica sol which were gelled, dried and aged at 600° C. Silica sol wasprepared from Si(EtO)₄ in ethanol, water and dilute mineral acid. Theresultant alumina/silica washcoat showed a surface area of 390 m² /g anda pore size of 45 Å. In a second experiment the alumina/silica washcoatwas prepared by treating an alumina washcoat with a silica sol,resulting in a pore size of between 30 and 40 Å. In another embodiment,alumina/silica materials were prepared from (tBuO)₃ Si--O--Al (OR)₂, andAl (OR)₃ in parent alcohol. These materials also retained a high surfacearea after aging at 1100° C. in air.

The alumina sol is preferably prepared by Burgraff's method ofpreparation of supported and unsupported alumina washcoats. Aluminumsec-butoxide is hydrolyzed in water and heated to remove isopropanol.The residue is treated with dilute nitric acid and peptized for sixteenhours to obtain boehemite sol. The surface area of bulk gel derived fromboehemite sol and fired to 600° C. is 200 m² /g and pore size is 45 Å.Notably, Burgraff reports a pore size of 60 Å while our experimentsyielded a pore size of 45 Å.

The concentration and viscosity of alumina sol is then adjusted to makeit suitable for deposition on the substrate or alternatively on one ofthe two membranes (in FIGS. 2 and 3) of prefabricated wafers bymicrocapillary device. The amount of liquid dispensed and the wettingcharacteristics of the fluid to the substrate determine the thickness ofthe alumina washcoat. In practice, it is difficult to generate verysmall drops, however, part of the liquid can be removed by suctionleaving only a controlled amount of alumina on the substrate. Afterdrying, the sol is slowly heated to 400° C. and cooled to roomtemperature, leaving a thin supported alumina washcoat on the substrate.

The alumina washcoat, prepared by this method, has low residual stress,is rather uniform with the exception of the border region and retainshigh surface area and porosity. The thickness of the alumina washcoatcan be varied in the 20-100 nm range and a thicker washcoat can bedeposited by repeated applications of alumina sol. An advantage of thismethod is that it results in an alumina washcoat having relativeuniformity and thinness which improves thermal contact between thealumina washcoat and the substrate.

Additionally, the ability to make a thin layer using the sol-gel processprovides good thermal contact between the catalytic layer and thetemperature measuring layer. A thin layer allows the temperaturemeasuring layer to accurately measure the heat produced by the catalyticlayer.

The preferred method of metal sol preparation includes the use of eitherplatinum or palladium as the metal of choice to produce particles havinga small and controlled size as described by Schmid.

Platinum or palladium sols are prepared by reduction of chloroplatinicor chloropalladic acids in water. The sols are then stabilized bytreatment with sodium salt of sulphanilic acid. After concentrating thesols, metal particles are isolated in solid state (particle size 50-200Å) and redissolved in water. Practically any concentration of metalparticles in water yields acceptable results. Metals are then depositedby microcapillary device on the alumina washcoat of the prefabricateddevice described above. No further processing is necessary, because theorganic component is lost when heated to the operating temperature. Theuniform small particle size of the metals and the controlled amountscontribute to a large number of catalytic sites on the sensor device.

The catalytic layer that would be utilized would preferably comprisesol-gel processed catalytically active metal particles in combinationwith a sol-gel processed washcoat. The metal particles can be depositedon the washcoat or mixed with the washcoat. The term "sol-gel processedcatalytic layer" includes: 1) a sol-gel processed washcoat together withsol-gel processed catalytically active metal particles; and 2) a sol-gelprocessed washcoat together with catalytically active metal particles.

The preferred method of providing a washcoat on a substrate, includes achanneled substrate, such as a honeycomb ceramic structure or the like.The disclosure of this method, in the U.S. Pat. No. 5,210,062 disclosedby Nauru et al., herein incorporated by reference, showed that thesol-gel process was suitable for deposition of a washcoat on a catalytichoneycomb substrate. The '062 patent also demonstrated that the sol-gelprocess reduced alumina buildup in the corners of the channelledsubstrate, thereby alleviating the back pressure problem which existedprior to the '062 patent.

The method disclosed in the '062 patent comprises first applying acoating of a reactive mixture on the substrate. A reactive mixture ismade by combining a certain type of aluminum alkoxide containinghydrolyzable alkoxy groups with water and acid, generally with stirring,wherein a suspension is formed. The aluminum alkoxide useful with thisinvention has the chemical formula: Al (OR)₃, wherein R comprises alkylgroup, branched alkyl group, or aryl group having between 3 and 6 carbonatoms. Aluminum alkoxides which may be used in this invention include,but are not limited to, ethoxides, (n-, or iso)propoxides, (n, sec, ortert-) butoxides, or (n, sec, or tert-) amyloxides. The excess coatingfrom the channels can be removed by blowing gas through the channels.The reactive coating is then hydrolyzed with the addition of water. Thecoating on the substrate is then dried at a temperature suitable toremove water present in the coating, preferably at or below about 100°C. The method also includes calcining the coating, preferably at atemperature greater than about 300° C., most preferably between about300° and 900° C., to densify the coating and convert it to γ-alumina.The method may additionally comprise repeatedly applying and drying thecoating followed by calcining or doing all three steps until a coatingof desired surface area is obtained.

The reactive mixture may further comprise other components such ascompatible salts of materials like barium and cerum which would alsoform oxides thereof in the washcoat. The presence of barium oxide andcerium oxide in the washcoat improves the high temperature stability ofthe washcoat and the oxidation efficiency of the catalytic layer duringuse.

For the same reasons, the above method provides a more efficientcatalyst for combustible sensors. As with catalytic converters, thecatalytic layer of the calorimetric sensor should be disposed on thesubstrate such that the catalytic active sites are maximized, and toprovide an unrestricted flow of exhaust gases to pass through thecatalytic layer.

In the above preferred method, the substrate is made preferably of asubstantially chemically inert, rigid solid material capable ofmaintaining its shape and strength at high temperatures. The substratemay be metallic or ceramic in nature or a combination thereof. Suitablematerials are α-alumina, cordierite, alpha-alumina, and zirconiumsilicate. The preferred substrate is the honeycomb ceramic structure.The preferred aluminum alkoxide comprises aluminum tris (sec-butoxide)and the preferred solvent is sec-butanol and the preferred sol isalumina/silica.

In sensor applications, it is also desirable to incorporate at least oneother metal atom in the aluminum oxide washcoat. For example, the U.S.Pat. No. 5,134,107 issued to Narula, herein incorporated by reference,teaches a method for making single phase lanthanide-aluminum-oxidematerials. Research has shown that when employing aluminum oxidematerials as a catalyst washcoat it is desirable to include lanthanum orcerium atoms or both in the aluminum oxide matrix. Incorporating eitheror both of those metal atoms in the aluminum matrix tends to preventstructural changes that occur in unstabilized γ-alumina at hightemperatures, which would tamper with the efficiency of a catalyticsensor. When using sol-gel techniques to make the alumina material,these other metal atoms are added by co-hydrolyzing one or moremetal-alkoxides with aluminum alkoxide.

Prior to the '107 patent, such alkoxides when combined in waterhydrolyzed, resulting in a mixture of hydroxides. The undesirable finalproduct of such a mixture comprises a non-uniform 2-phase distributionof metal oxide in an aluminum oxide matrix. To overcome thesedisadvantages, the '107 patent teaches a method which comprisesreacting, according to sol-gel techniques, water and heterobimetallicalkoxides comprising tribis(2-propanolato) alumina)hexakis-(2-propanolato))!lanthanide represented by the general chemicalformula Ln Al(OPri)₄ !₃, Ln being a lanthanide. Lanthanide is meant toinclude the members of the lanthanide series of the periodic table suchas lanthanum and cerium.

The lanthanide-aluminum-oxide materials according to the presentinvention are made from single phase sols. The sol may be made byforming a reaction mixture of the heterobimetallic alkoxides with water,and adding acid to the reaction mixture to form a sol. Acids employedembodiments of the present invention may be selected from any organicand inorganic acids which may include, but are not limited to, nitric,hydrochloric, sulfuric, acetic and propionic acid. Alcohol is generallyemployed as a solvent for the alkoxide prior to it being combined withwater. Alcohols which may be broadly employed according to embodimentsof the present invention include 2-propanol, n-butanol and sec-butanol,with 2 propanol being preferred. The preferred heterobimetallic peroxideis tris (bis2-propanolato)alumina)hexakis(α-)2-propanolato))! lanthanum.The sol is preferably stabilized by maintaining the reaction mixture fora time and a temperature sufficient to form a stable sol. A stable solis one that maintains its sol properties and does not experience anysubstantial gelling when exposed to air or moisture for a significantperiod of time, e.g., months.

To form a washcoat, the sol is coated on the substrate and then thecoating is dried and subsequently calcined at an elevated temperature.Generally calcination is carried out at a temperature above 300° C.,preferably between about 300° C. and 900° C. to form alanthanide-aluminum-oxide material.

Rather than forming a gel from the sol above, gels may be made moredirectly from lanthanum aluminum alkoxide. For example, the addition ofa wet alcohol, generally meant to be one containing more than sixequivalents of water, to a solution of the alkoxide in an alcohol atroom temperature results in gel formation instantaneously at the contactlayer. These sol-gel techniques may also be employed to make aluminummaterials comprising more than one lanthanide as a single phasematerial.

Further, the U.S. Pat. 5,234,881 issued to Narula et al., hereinincorporated by reference, discloses a method of making binarylanthanum-palladium oxides useful as an automotive exhaust catalystwashcoat at high temperatures. The teachings of the '881 patent can bereadily applied to catalytic sensors.

Prior to the '881 patent efforts to deposit the binarylanthanum-palladium oxide catalysts from their suspension in waterfollowed by sintering resulted in the loss oflanthanum-palladium-oxides. The above-problem was solved in the '881patent by depositing such oxides from their suspension in an alumina solon a honeycomb substrate precoated with a commercial washcoat such asγ-alumina. The alumina sol for this purpose can be readily prepared byhydrolyzing aluminum sec-butoxide in water at 70° to 90° C., boiling offsec-butanol at 90°0 C. and acidifying. Two different samples were madeby suspending the individual binary oxides (La₂ Pd₂ O₅ or La₄ PdO₇) insuch sols in depositing such suspension onto catalyst substrate, such asmonolithic cellular cordierite, which has been previously coated with acommercial washcoat such as γ-alumina. On drying, alumina sol forms agel and traps the particles of the binary lanthanum-palladium-oxide. Thecatalyst is then sintered, preferably at 600° C.

For the '881 patent other suitable sols such as SiO₂, TiO₂ and ZrO₂ canbe substituted for an alumina sol, Al₂ O.

Alumina sols were also prepared in the '881 patent by hydrolyzingaluminum alkoxide (eg. Al(OR₃) in water followed by peptization in thepresence of diluted mineral acid. The sol was then concentrated to a gelwhich was heated at between 250° C. and 600°0 C. in a nitrogenatmosphere. Alumina/silica washcoats were prepared for a mixture ofalumina sol and silica sol which is obtained from Si(ETO)₄ in ethanol,water, and diluted mineral acid. A sample of alumina/silica washcoat wasthen made with uniform pore size around 45 Å and 390 m² /g.

Further, the sol-gel processed washcoat, can also include as thecatalytically active metal, transition-metals. When testedexperimentally, silver-containing sol, such as AgNO₃ which is watersoluble was dissolved in distilled water. The silver solution was thenused to impregnate the sol-gel washcoat. The ratio of the silver amountto the sol-gel weight was dependent on the desired silver loading on thesol-gel washcoat. After the impregnation, the material was dried up to120° C. and then heated in air inside a furnace of 500° to 600° C. forfour hours.

Additionally, the present invention's preferred method for fabricating asensitivity enhanced calorimetric device, includes usingsilicon-micromachining. Silicon micromachining offers the capability offabricating devices with low power consumption at potentially lowermanufacturing costs. A micromachined device can also have a fasterresponse time because the membrane temperature measuring layer and thecatalytic layer have a smaller thermal mass than conventionalcalorimetric devices.

The preferred embodiment includes fabrication of a silicon micromachineddifferential microcalorimeter. To improve the detection limit of thesensor the temperature rise is preferably measured differentially byadding a second element with thermal characteristics identical to thoseof the temperature measuring device, but without a catalytic layer. Theresistances of the two elements are represented by the followingequation:

    R.sub.catalytic =R.sub.o  1+α(T+ΔT.sub.comb !

    R.sub.reference =R.sub.o  1+αT!

with R_(o) the resistance at 0° C., α the temperature coefficient ofresistance, T the temperature of operation in ° C. and ΔT_(comb) therise in temperature caused by the oxidation of combustible gases on thecatalytic layer, ΔT_(comb) is given by:

    ΔT.sub.comb =(R.sub.catalytic -R.sub.reference)/ΔR.sub.o =ΔR/αR.sub.o

with ΔT_(comb) /1000 ppm of combustible gas defined as the sensitivityof the sensor.

FIG. 2 shows a perspective view and FIG. 3 shows a schematiccross-sectional diagram of one of the device configurations fabricatedand studied. Two thin-film resistors are fabricated on two micromachinedmembranes of low thermal conductivity, and one is covered by a catalyticlayer. For simplicity, no heater was incorporated in the design and thedevices were heated externally.

The average temperature rise in a microcalorimeter is dictated by thebalance of heat produced by the chemical reaction and the heat lost tothe environment. In order to maximize the detection limit of the sensor,effects such as the reactant mass transfer, the reaction kinetics at thecatalyst, the heat loss by conduction/convection to the ambient gas andby conduction to the substrate, thermal fluctuations in the environment,and the electrical characteristics of the thermometer must all be takeninto account.

The key elements of a Si-based microcalorimeter are the catalytic layer,the temperature measuring layer, the heater, and the supportingstructure or substrate for all of the previous elements. The substrateconsists of a bulk silicon frame with either a membrane layer or a morecomplex plate/teeter element which in both cases are obtained by etchingthe underlying bulk silicon frame. The membrane or plate/teeter acts asa support for the temperature measuring layer. Multiple silicon dies canbe fabricated from a single silicon wafer. The membrane or plate/teetershould have a small thermal mass for fast response time, but must bemechanically robust to support the temperature measuring layer and thecatalytic layer and survive temperature cycling, pressure shocks, watermist and small particle impingement. It should also be configured insuch a way as to minimize the heat loss to the silicon frame and to theambient gas for increased sensitivity. The catalytic layer should have alarge specific surface area for the device to operate in mass-transportlimited regime. This surface area is achieved by using sol-gel processedalumina-silica washcoat and/or sol-gel processed catalytically activemetal particles.

Additionally, good thermal contact between the catalytic layer and theunderlying temperature measuring layer is also important for increasedsensitivity. The catalytic layer should not substantially change thethermal characteristics of the membrane, otherwise the sensortemperature compensation may be compromised. For greater sensitivity,the temperature measuring layer should mainly measure the central regionof the membrane where the temperature rise due to the reaction is thelargest, without substantially contributing to conductive heat loss. Athin-film resistor with stable resistance and temperature coefficient ofresistance (TCR) is desirable as the temperature measuring layer. Thefilm resistor is patterned as a winding element to increase itsresistance (i.e., the output signal) and distribute the stress inducedby the thermal mismatch with the membrane.

There are three ways to process the sol-gel catalytic layer. One methodincludes using a micro syringe to deposit the catalytic layer onto themembrane. With this method, the catalytic layer is deposited to createcatalytic active sites specifically at the desired locations. Althoughthis technique provides accurate deposition of the catalytic layer, thistechnique may not be suitable for mass manufacturing.

A second method involves dipping the silicon wafer into a sol-gelsolution to coat the silicon die with a sol-gel processed catalyticlayer. This second method is preferred as it provides a controllable andefficient method to batch fabricate catalytic calorimetric gas sensorsin a way that is compatible with silicon micromachined structures. Thethickness of the catalytic layer can be readily controlled by varyingthe speed with which the silicon wafer is immersed and removed from thesol-gel solution. This dipping method further requires removal of thesol-gel catalytic layer from specific areas to create the catalyticactive sites at the specific desired locations. Selective removal of thecatalytic layer can be effectuated in two ways. One involves fabricatinga mask on the silicon wafer and placing the mask over the substratewhich is then followed by etching away the sol-gel catalytic layer fromthe undesired locations. The second involves heating the catalytic layerby resistive heating of the membrane on which a catalytic active layeris desired such that the solvents within the sol-gel catalytic layerburn off, resulting in the affixation of the sol-gel catalytic layer inthe desired areas. This step is then followed by washing the sol-gelcatalytic layer with a solvent to strip the remaining sol-gel solutionfrom the catalytic layer. An acid solvent could strip the solution fromthe undesired locations. The result in either case is a silicon waferhaving a sol-gel processed catalytic layer selectively placed thereonthat is easily reproducible. At this time, the preferred method includesdipping the silicon wafer and then etching away the sol-gel solutionfrom the undesired locations.

With the preferred method of fabricating a silicon microcalorimeter, themembrane is deposited first on a silicon wafer, 100 mm in diameter, 400μm thick. The membrane is in total preferably between 500-1000 nm, suchthat it is mechanically stable while being thin enough to prevent lossof heat by thermal conduction through the substrate. Either a 0.6 μmthick layer of low-stress, low pressure chemical vapor deposition(LPCVD) silicon nitride, or a composite of plasma enhanced chemicalvapor deposition (PECVD) silicon oxide/nitride layers (about 0.5 μm and0.1 μm, respectively) deposited over 0.1 μm of LPCVD nitride can beused. After annealing at 600° C., the latter composite layer has a smallresidual stress (tensile) of about 6×10⁸ dynes/cm², the compressivestate of the oxide being compensated by the tensile nitride. A Pt filmresistance thermometer, 100 nm thick, is sputter deposited. The filmresistors, acting as temperature measuring devices and/or heaters, aredelineated by lithography and wet etching. After annealing the Ptresistors at 500° C. in nitrogen to stabilize their resistance andtemperature coefficient of resistance (TCR), the wafers are coated with0.2-0.3 μm of PECVD silicon nitride for passivation and annealed at 500°C. The passivation is then removed on the contact pads with plasmaetching. While defining the opening for the contact pads, an etch-maskpattern is also defined on the back side of the wafer using adouble-sided aligner. A 30% aqueous solution of KOH at 80° C. is used tocompletely etch the silicon underneath the membrane. The membrane hassufficient mechanical strength to allow the wafer to be diced with adiamond saw. For ease of handling, a die size of 7×7 mm² is used,although only a 3.5×3.5 mm² area is needed for the device with thesmallest membrane.

While the best mode for carrying out the invention has been described indetail, those familiar with the art to which this invention relates willrecognize various alternative designs and embodiments for practicing theinvention as defined by the following claims.

We claim:
 1. A differential gas combustible microcalorimeter sensorcomprising:a substrate which comprises:a silicon frame, a firstreference membrane disposed on said silicon frame, and a second membranedisposed on said silicon frame, said second membrane being positionedsuch that said second membrane is thermally isolated from said firstreference membrane; a first and second temperature measuring devices,the first temperature measuring device being disposed on said firstreference membrane and the second temperature measuring device beingdisposed on said second membrane; and a sol-gel processed catalyticlayer disposed on said second temperature measuring device, wherein saidfirst and second temperature measuring devices provide a measurement ofthe additional heat generated by the catalytic layer to provide a meansof calculating the temperature fluctuation in a combustible gas thatcomes into contact with the sensor.
 2. The sensor of claim 1, whereinsaid sol-gel processed catalytic layer comprises:a sol-gel processedwashcoat, wherein the washcoat is selected from the group consisting ofalumina, silica and mixtures thereof; and a plurality of sol-gelprocessed catalytically active metal particles deposited on said sol-gelwashcoat.
 3. The sensor of claim 2, wherein said catalytically activemetal particles are noble metals selected from the group consisting ofplatinum, palladium, silver, gold, ruthenium, rhodium, osmium, iridium,and mixtures thereof.
 4. The sensor of claim 1, wherein said sol-gelprocessed catalytic layer comprises:a sol-gel processed washcoat,wherein said washcoat is selected from the group consisting of alumina,silica and mixtures thereof; and a plurality of catalytically activemetal particles deposited on said sol-gel processed washcoat.
 5. Thesensor of claim 4, wherein said catalytically active metal particles arenoble metals selected from the group consisting of platinum, palladium,silver, gold, ruthenium, rhodium, osmium, iridium, and mixtures thereof.